I'm glad to hear that you plan on keeping your older girls until they pass away. My two oldest are sisters and they're going on 15 years! I keep thinking every year will be their last but they just keep going. But remember - aging chickens are just like aging humans. As the body ages, things start to not work so well (including the immune system, the digestive system, eyesight, hearing), arthritis can set in, balance can become a problem.
Most humans unfortunately do not pass away peacefully in their sleep and chickens are the same way. Because you find a chicken dead on the floor of the coop in the morning doesn't mean it died peacefully in its sleep and then fell off. I've had older hens with balance issues who in the night fell from their perch and the impact of hitting the ground caused a fatal injury that killed her (ruptured organ or punctured lung). I've learned to make adjustments for older hens like ramps to lower perches for them, nest boxes on the ground that they can sleep in instead, etc.
You must be prepared to assess the quality of life of your aging chickens and be prepared to euthanize them when you see signs of failing health (loss of appetite or weight, painful joints, etc). It's very hard to make the decision that the time has come to say goodbye but just waiting for them to die on their own can sometimes be a long and painful wait for your chicken.